The adaptive array antenna technology for directive transmission/reception is known as an art that realizes to increase traffic capacity, expand communications areas, suppress interference, along with the increasing demands for the mobile communications systems. The adaptive array antenna technology is an art having an array antenna constituted by a plurality of antenna elements in order to suitably place transmission/reception directivity under control by the use of the signals received by the array antenna, together with the other pieces of information.
Studies have been made on a variety of approaches to the directivity control on the adaptive array antennas. For example, there is an approach to estimating the arrival direction of a reception signal such that directivity is placed under control toward the estimated direction. In this approach, it is known that, where there is a deviation in amplitude/phase on the array antenna at between the reception branches, there encounters a deterioration in the accuracy of direction estimation and distortion in the pattern of directivity. This results in an impossibility to secure a desired effect.
The amplitude/phase deviation at between reception branches results from the characteristic difference between analog elements of a radio circuit section. Hence, there is a difficulty in suppressing the occurrence of such deviations. Meanwhile, the amplitude/phase deviations vary from time to time due to the effect of aging, temperature change and so on. Thus, there is a need for a calibration technique for correcting for amplitude/phase deviations occurred between the reception branches, moreover a calibration technique for correcting for deviations varying in real time without requiring to shut down the communications system.
The configuration described in JP-A-2002-77016 is known as a prior art concerning calibration technique. In this configuration, the signals received at the array antenna are added with by a reference signal generated by a reference signal generator at the inside of a base station. Then, the reference signal is removed from the signal sent through the radio circuit section of each reception branch, to detect information of an amplitude-and-phase deviation over the reception branch. The detected information is used to effect calibration.
Meanwhile, there is a calibration technique using only reception signals without applying interference signals, in “Background Calibration Method for Array Antenna and its Performance” by Keiji Takakusaki, et al (IEICE Technical Report, RCS2001-261, March 2002). According to the document, a calibration branch based on a radio circuit section 1003C is provided for reception branches based on radio circuit sections 1003-1, 1003-2, . . . , 1003-N, as shown in FIG. 1. The signals received at each Of the antenna elements of the array antenna 1001 are used, to detect channel estimation values on the reception branches by a correlator 1005-1 and channel estimating unit 1006-1 and on the calibration branch by a correlator 100SC-1 and channel estimating unit 1006C-1. A correction-value detecting unit 1010 compares between the channel estimation values, thereby detecting amplitude/phase information on the reception branches. Using the detection information, correction values are read out from a correction-value memory unit 1011, thereby calibrating the reception signals in an inter-array signal compositing unit 1012. Selector units 1004-1,1004-2 selects a signal from a predetermined antenna element of the array antenna 1001, under control by a selector control unit 1013.
However, in the related art described in JP-A-2002-77016, the reference signal acts as a signal interfering with reception signals. This results in reception characteristic deterioration.
Meanwhile, in the related art of FIG. 1, where the reception signal has a small power, the power on the calibration branch is rendered further smaller, thereby deteriorating the accuracy in the channel estimation value on the calibration branch. This causes deterioration in the accuracy of a correction value computed by using the relevant channel estimation value.